I wrote this supplemental: [ http://www.twinturbo.net/nissan/300zx/forums/main/view/866841/Supplemental-Tips----Heater-Core-Replacement.html ]
A general web search will probably turn up others, it was a pretty popular issue in the mid-2000s. I'd also recommend while fixing the motor you also consider doing the core since everything will be pulled apart.
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